1. Field of the Invention
This invention relates to digital-to-analogue converters, and particularly to converters which use an array of current sources to provide an analogue output.
2. Related Art
Digital-to-analogue converters of this type are widely used in numerous applications, and can be implemented as high speed converters suitable for use in telecommunication systems. In one arrangement, the array of current sources is arranged as a plurality of identical current sources. An associated switching array selectively routes the current source outputs to two possible output terminals. In some systems, one of these terminals is simply ground, and the other terminal is a current summing bus. The signal on the current summing bus is converted into a voltage before being provided as an output of the converter. In other systems, the output of the converter is to be provided on to twisted pair cabling. In this case, the current provided to one of the output terminals is converted to a voltage for one of the twisted pair wires, and the current provided to the other terminal is converted to a voltage for the other wire of the twisted pair. In this case, the converter output is represented as the difference between the voltage levels on the two wires of the twisted pair.
It has been recognised that to provide high precision, for example when converting more than 10 bits, it is necessary to calibrate the current sources constantly during the conversion process.
U.S. Pat. No. 5,870,044 discloses one circuit and method for providing calibration of the current sources used in the converter array. Essentially, an additional current source is provided, and the output of the additional current source is used to replace a current source within the array when that particular current source within the array is being calibrated. In this way, all current sources within the array can be calibrated in a cyclical manner. The additional current source avoids the need to interrupt the conversion process during calibration.
The calibration of individual current sources enables compensation for variations in layer thicknesses or other dimensions across the current source array. These variations across the array are static errors. There are also dynamic errors which result from the various parasitic capacitances within the current source circuits. After calibration of a current source to provide a desired output, changes in the charges stored on these parasitic capacitances vary the current source output over time. There is therefore also a need to carry out a continuous cyclical calibration process.